Ketul, if your working with SAN and used cloning/mirror/copy(Gold Image, etc...) of a LUN Volume that volume may be readonly at the SAN software level. Also make sure your disk is online for this operation. If the readonly flag on the disk is still set to readonly that may indecate that SAN is not allowing that flag to be cleared.

On my disk (external USB 3.0 dock, 3.5" SATA 320GB HDD), when I tried Diskpart, it listed it as CLEAR of 'Read-Only' attributes, but I still had to run the 'attrib vol clear readonly' command. Then it has allowed me to format the volume (it allowed me to create a volume after allowing me to delete the previous volume but gave the 'read-only' error message at the point of trying to format it). It's currently formatted 7% so progress has been made.